Diminishing oil reserves, Governmental incentives and increased awareness of environmental issues have seen a rise in the use and desirability of generating power from renewable energy sources such as wind, solar and hydro energy. However, whilst renewable energy sources often have advantages in terms of sustainability and environmental impact, in many cases they also suffer from disadvantages such as unpredictability in supply. For example, generating energy by harnessing wind or solar energy is limited to when the wind blows or the sun shines brightly. These situations are subject to a high degree of uncertainty, especially in time scales of the order of a week or greater. This leads to difficulties in maintaining a constant supply to the electricity grid and may lead to difficulties in scheduling power generation resources and the need for standby generation capacity.
In recent years, one of the most high profile forms of renewable energy has been wind power. Wind turbines are used to harness wind power to generate electricity. Wind turbines generally have two or more open blades, optimised for rotation by wind and mounted to a shaft that drives a generator via a gearbox. Typically, the blades are mounted high on a support. To obtain utility levels of power, several turbines are often grouped together. Thus, wind turbines used for power production on a large scale are generally large, visually intrusive structures and often located in areas of natural beauty. As such, wind turbine developments are often subject to objections by local residents, environmentalists and natural heritage groups.
Hydropower is another well-known form of renewable energy. This may be extracted via a range of technologies. These include the damming of rivers to allow a controlled release of the water through turbines and the use of tidal dams that allow water to flow through the dam via sluices during rising tides and stored water to flow through turbines during low tides. However, these methods suffer from a range of problems including high construction costs, a lack of suitable sites and adverse environmental impact due to the need for sizable structures and flooding of land.
Another form of hydropower is wave power. This may be harnessed by various means, including funneling devices that channel waves up into a chamber either driving a turbine directly or causing air flow that drives the turbine. Another approach is to deploy a multi-segmented floating structure that uses the relative movement between segments caused by waves to actuate a piston that drives a generator. These methods also suffer from drawbacks, including a low capture efficiency and exposure to potentially damaging storms and extreme sea conditions.
One promising means of hydropower extraction is through the use of water turbines. These are devices that utilise blades that are turned by water flow, for example in rivers or due to tidal currents. An advantage of this approach is that power generation equipment can often be submerged or sited out at sea, away from habitation to avoid visual impact. As such, movement of water in the form of tidal currents is an ideal and under-exploited source of renewable energy.
Conventional water turbine technology typically uses a turbine having a single rotor, each rotor having two or more blades, arranged so as to rotate under the action of water currents. The rotor may be open or enclosed within a cowl. Power take-off is generally by means of a shaft coupled to the rotor that drives a conventional generator. Alternately, the turbine can be used to pressurize a hydraulic fluid to drive a hydraulic motor that in turn operates an electrical generator. The speed of rotor shaft rotation may be controlled to the optimum generator speed by providing a gearbox between the rotor and the generator. Turbines may be hung below floating pontoons or be anchored to the sea floor or riverbeds.
Despite the advantages of using tidal and river power, there are some challenges that must be overcome in exploiting this energy source. As water is denser than air, water turbines generally tend to be slower turning and are subjected to higher torque than equivalent wind turbines. In order to operate a generator at an efficient speed, the rotational speed of the water turbine generally needs to be increased through the use of gearing. Due to the high torque, the required gearings are generally heavy and inefficient. In addition, they add excess weight and complexity to the system and require increased maintenance. Also, operation of conventional water turbines can create a turbulent downstream flow of water, akin to a wake from a boat. This can lead to a scouring of the sea floor or riverbed downstream of the turbine.
Maintenance of water turbines can also be problematic due to the detrimental effect of fouling caused by the growth of waterborne organisms on the system and the corrosive effect of salt water. These problems are compounded by the difficulty of turbine access. Installation and removal of these devices can also be problematic. Due to the high torques experienced by water turbines, firm securing means, such as piling, are generally used. These require a high degree of installation time, are costly and can have an adverse environmental impact. An alternative approach is to use a heavily weighted base. However, this is unsuitable for systems having a high reactive torque, which may result in movement of the turbine installation.
An object of the present invention is to provide a solution to at least one of the above problems.